The Enertia House

May 19, 2007

I recently got a query from one of the editors of Mother Earth News regarding a news story she had read in the New York Times. The writer, David Pogue, had been a judge in a contest sponsored by by the History Channel and the National Inventors Hall of Fame titled “Modern Marvels/Invent Now.” A $25,000 prize was awarded to one amoung 25,000 contestants, and the winner was the Enertia House, which was invented by engineer and former log-home architect, Michael Sykes.

The Mother Earth News editor said that these homes had been featured in their magazine before. They essentially provide two wooden shells for the home, one inside the other. She said that there was no mention in the article about the cost per square foot. She was wondering what I thought about the concept from the standpoint of sustainable architecture.

Here is my response:

Double envelope house designs have been around for several decades and they definitely offer some benefits, as well as raise some questions. Any house that takes advantage of the geothermal properties of the ground will be doing its inhabitants and the earth a good turn. This can take the form of earth-sheltering in general, or some clever system of circulating air like the Enertia concept; coupled with sensible passive solar design, it is possible to approach a “zero energy” home.

The concerns about their system that I have are: The use of wood as the primary building material is not generally sustainable in this day of lost forests. With the double envelope design, you are practically building two houses to end up with one. Relying on wood as a thermal mass material compromises the potential thermal performance because wood does not serve this function nearly as well as traditional masonry thermal mass materials. So, I guess what I am saying is that a more sustainable and less costly design can be accomplished in more traditional ways.

Answering the same question, Paul Scheckel wrote, “At first glance, this looks a lot like sunspace design from the 70s (without the stone-filled basement to store heat) which overheated in the daytime and lost lots of heat at night. Consider also that this giant convection oven requires a temperature difference, which in this case is driven by the sun and the cool basement. A New England winter has precious little sun, so my heating system will drive the convective loop, increasing heat loss (in addition to the insulation-free envelope). I haven’t heard too many people (ie: none) say that wood is bad for houses and better for biodiesel, but there are good arguments for not using so much material in a home. Does it work? I’d like to see one built in the northeast and see the resulting energy data, wherein the proof will lie.”

Clark Snell of wrote, “I spent five minutes looking over the web site, so these comments are only based at looking at marketing materials, i.e. they may be inaccurate. Ditto what has been said so far. A couple more “red flags:”

  1. Solid wood envelope. They seem to be using the old “mass enhanced R-value” argument for why solid wood walls perform well thermally. I think it’s well established that this is true only in very specific climatic situations. Touting solid southern yellow pine walls in comparison to solid white pine walls is like saying a Chevy Suburban gets better gas mileage than a Hummer…that’s not really a useful statistic.
  2. Energy without oil. The presentation intimates that this is a completely passive design. For example, no heating system is mentioned. That simply isn’t credible for most climates using the technology they are describing.
  3. Passive means local. You simply can’t create a design that relies heavily on passive techniques and generalize it across climates. In my area where we have high humidity, I’d wonder about this convective loop through the attic and basement, for example.

I could go on. I’m a passive design freak, so I’m all for the basic concepts they are dealing with. However, I don’t see anything really new here, but see marketing claims touting what they are doing as a major breakthrough and “the answer”. That always makes me nervous.”

David Eisenberg, of wrote, “After a skimming around their website, I see that they sell kits and their

base prices don’t include a lot of things - some of which are enumerated:

“Enertia Homes are sold as pre-cut, numbered kits varying in size from 1000-6000 square feet. The kit is a structural package that includes the timbers for the four exterior walls and the two interior walls (Energy WallsTM) which form the envelope, as well as the flashings, gasket, spline and fasteners to put the structure together. Also included are the beams for the upstairs floor system and the rafters for the roof structure. Doors, windows, flooring, and foam SIP roof panels are priced separately as per your blueprint and climate.”
That’s a pricey list of not includeds and notice they say nothing here and I saw nothing in my quick scan of the site about some really big and typical costs like excavation and foundations, below grade walls, or basement floor. They say this is a structural package but they don’t mention all the things that are going to be extra that most people would expect in a house - plumbing, wiring, fixtures (electrical and plumbing), stove, etc. and especially that the solar PV and thermal water heating systems are not part of the package. It would be nice if they said right up front and clearly what they do and don’t sell. And they should make it very clear that all the prices include only the factory labor, not the cost of actually assembling and finishing these structures.

But the biggest issue I have is that these are essentially double wall structures using an enormous amount of thick, milled lumber, which appears to use many times more wood than goes into a stick frame house. It would be interesting to see if they use more wood than a comparable log home. They’ll likely be more energy efficient than a log home, but they’ll use as much or more wood. Which raises all sorts of issues about the sustainability of this venture - beyond just the trees cut down - much bigger transportation, milling, probably kiln drying impacts as well. The concept is fine and likely works reasonably well in most climates. I’d need to see much more actual performance data and of course real cost data to be able to make any kind of realistic judgment of the viability of this concept as anything more than a niche market system. But between the costs which are going to be very high and the amount of materials going into one of these, calling it sustainable seems like a real stretch.”

And finally, Jeff Judkoff of the National Renewable Energy Laboratory, wrote: “The concept of “Double Envelope” homes has been around since at least the 1970’s. A number of them were built in the late 70’s early 80’s. Some variations of the concept were published in the Solar Home Book, by Bruce Anderson and Michael Riordan in 1976, Cheshire Books. Other publications in that time frame also showed the concept. The only truly new concept here is the notion that the resins in the wood behave as phase change storage materials. I have no idea if that is true, but I doubt it because the most common phase change is from a solid to a liquid, in which case the resins in their liquid phase would leak out creating a mess. That’s not to rule out the possibility that some tree resins could go from a solid to semi-solid phase, or that they are encapsulated in the wood, I just don’t know if they can, and would only be able to determine it through controlled scientific testing in a calorimetry chamber. Phase change storage can really be a big boost to the performance of many flavors of passively heated and cooled homes.

There are many ways to acheive highly efficient homes that more or less “heat and cool” themselves. Different approaches have different costs and will work better in some climates than in others. In Colorado, my lab, NREL, worked with Habitat for Humanity to create a net energy producing home. We used super insulation, passive solar tempering, ventilation heat recovery, engineered shading, solar hot water with a backup instantaneous water heater, compact flourescent lighting, and PV. We also have more than a years worth of detailed data to prove the performance of the home (it really was a net energy producer for the last year).

I saw no data to indicate how well the Enertia home actually performed from an energy perspective. Cost, energy performance, and comfort are the key criteria by which to evaluate such homes, and data is always better than arm waving, or catchy theories. Nothing beats the scientific method for objectively determining the value of an idea.”

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